If it's an older model with an adjustable main jet on the carb, I'd say the screw needs to be backed out a little. Tecumseh's act like that.

Otherwise, you have some varnish in the main jet, most likely.

Surging is almost always caused by a lean fuel mixture. If the carb does not have an adjustable jet, it is time to disassemble it and clean it with solvent and compressed air.